Thin film photovoltaics (PV) based on Cu(InxGa1-x)Se1 (CIGS) and similar technologies such as Cu2ZnSnS4 (CZTS) or Cu2ZnSnSe4 (CZTSe) are promising candidates for low-cost, high-efficiency solar cell applications. CIGS technology has demonstrated the highest energy conversion efficiency among all thin film PV technologies. The most commonly used substrate for CIGS, CZTS, CZTSe or similar PV cells is inexpensive and readily available soda lime glass (SLG) coated with a molybdenum thin film as the back metal contact. The molybdenum layer also reflects unabsorbed light back into the PV absorber layers.
An important process associated with the above classes of solar cells is sodium (Na) diffusion from the SLG substrate through the molybdenum back contact layer into the CIGS or similar absorber layer. Devices are fabricated by first depositing a thin film of molybdenum, typically about 500 nm in thickness, on a sheet of SLG, followed by deposition of the CIGS active layer onto the molybdenum film. Sodium diffuses from the SLG, through the molybdenum, and into the CIGS layer. This sodium acts as an electronic dopant in the CIGS layer and may have an impact on the device performance and final conversion efficiency.
The properties of the molybdenum film may play a role in determining the extent and characteristics of sodium diffusion from the SLG substrate into the absorber layer of a solar cell. Proper sodium concentration in the absorber layer may help to optimize the performance of a solar cell based on CIGS or similar technologies. At this time, the factors controlling sodium diffusion are not well known. Thus, process variations in the deposition of the molybdenum layer can produce what appear to be uncontrolled variations in device performance. Typically, CIGS cell manufacturers have assumed that the molybdenum films are constant in density and thus it is only the thickness of the molybdenum film that determines the extent and characteristics of sodium diffusion. This assumption has lead to the use of X-ray fluorescence (XRF) as the standard diagnostic measure of molybdenum films in CIGS or similar solar cell manufacturing.
This reliance upon molybdenum layer thickness as a controlled device fabrication parameter may cause production problems however, since most CIGS PV manufacturers are not aware that relatively small variations in molybdenum deposition conditions can cause large variations in the amount of sodium diffusion into the CIGS film. The unexpected and unknown variations in sodium diffusion can interact with and amplify variations in subsequent processing steps. This in turn may lead to reductions in manufacturing yields because of uncontrolled and unexplained variations in CIGS film properties and device performance.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.